Transient microwave conductivity studies of exciton dissociation in conjugated polymers

Photovoltaic solar cells based on organic systems are considered an emerging and viable technology, with respectable device performance characteristics and lifetimes. Common to almost all of these new devices is a nanostructured interface that comprises a donor, often a conjugated polymer such as poly (3-hexylthiophene), and more often than not, C60 as an acceptor. A unique and essential feature of these interfaces is the ability to efficiently dissociate the photo-generated excitons into free carriers and, more importantly, to very effectively inhibit the reverse, recombination process. A uniform consensus on why this happens has yet to emerge and it is therefore a topic of great interest. Although they are considered to be a viable technology, solar cell performances are struggling to exceed consistently 5% power conversion efficiencies, and there is a clearly a need to increase significantly this value. To do so, requires more fundamental, basic research to be focused on the problem, and it is this approach that motivates the presentation. There will be a focus on the use of pulsed-laser, time-resolved microwave conductivity as a tool for probing both the production and loss of free carriers that result from exciton dissociation. In addition to foundational data on the ubiquitous P3HT:PCBM system, a number of other systems will be discussed that include: replacing the PCBM with single-wall carbon nanotubes, and colloidal quantum dots; as well as changing the P3HT for new thiophene-related polymers. The emphasis of the discussion will be placed on examining the role that the varying energetics and differing interfaces play in controlling the kinetics of exciton dissociation and carrier recombination. Finally, the presentation will conclude with a discussion of how these basic studies might be used to improve solar cell device performances.